Offset printing is a process in which printing ink is transferred or "offset" from a printing plate to a rubber blanket and from there to a paper surface. When printing halftone dots by this process, a so-called "mechanical dot-gain" occurs because of the resiliency of the blanket. Ink is squeezed onto the surface, giving the dot a larger diameter than the corresponding dot on the printing plate. The amount of mechanical dot gain depends on printing conditions such as press pressure, ink rheology and paper quality. A so-called "optical dot gain" also occurs. The dot appears larger than it is, because light passing through the dot and paper surface is reflected within the paper at points outside the dot circumference ("shadow effect"). Amount of optical dot gain depends on paper texture and opacity.
Dot gain is lower at either end of the tonal range than at mid-range. It is ordinarily measured in print areas correspponding to 50% dot areas on the halftone negative from which the printing plate was made. Percentage values of halftone dot area coverage relate to a solid density of 100%. If the effective dot area (EDA) is 70%, then the dot gain is 20%. EDA is calculated by the Murray-Davies equation: ##EQU1## where D.sub.T is density of the original 50% dot area and D.sub.S is density of a solid area, both relative to the paper, as determined on a densitometer.
Web presses typically produce a high dot gain of 25-30, sheet-fed presses a medium gain of 20-25, and proofing presses a low gain of 15-20.
Off-press positive surprint color proofs used in the graphic arts industry are made by the process of Chu et al. U.S. Pat. No. 3,649,268. In this process a tacky photopolymer film is laminated to paper and exposed to ultraviolet light through a color separation to halftone-positive transparency, causing polymerization and reduced tack in exposed areas. Toner particles of corresponding color are applied to the exposed film and adhere to the tacky unexposed areas only. Non-adhered toner is removed, leaving a positive toner image on the unexposed areas. Another piece of film is laminated to the toned layer, and the process is repeated using a different color separation transparency and corresponding color toner. Typically, four layers are built up in this manner, using yellow, magenta, cyan and black toners, although the colors and number of layers can vary. Photopolymer films disclosed in the Chu et al. patent comprise a support layer and a photopolymerizable layer having a binder component, an ethylenically unsaturated monomer component, and a photopolymerization initiator.
In making a color proof, the proper amount of exposure can be determined using a positive transparency having discrete areas of different percent dots, e.g., 0.5%, 1%, 2%. Typically, exposure is set to reproduce ("hold") 2% dots but lose 0.5% and 1% dots due to overexposure. Under these conditions mid-tone dots are typically reduced slightly, so there is no mechanical dot gain. However, color proofs exhibit high optical dot gain, due to light penetrating the dot and photopolymer surface and being reflected from points below, outside the dot circumference ("shadow effect"). The amount of gain varies with the particular photopolymer system used (intrinsic gain) and the thickness of the photopolymer layer (apparent gain). Optical gain in a color proof is desirable, because the purpose of the proof is to simulate as closely as possible the results which will be obtained on the printing press.
Photopolymer systems disclosed in the Chu et al. patent produce films with optical gain of about 20% of more. Thus, they are entirely adequate for making proofs for web presses and for some sheet-fed printing, but a film with lower dot gain would be desirable for "sharper" proofing and printing applications such as sheet-fed and press proof printing on high quality paper.
Systems of Chu et al. produce high optical dot gain in part because they must be coated in relatively thick layers (70 to 110 mg/sq. dm. dry coating weight, about 6.5 to 10.5 microns dry thickness). Otherwise, it is difficult to achieve consistent results in preparing multiple proofs of the same scene, because the optical density obtained will be dependent upon how much time elapses between exposure and toning, due to migration of mobile components in unexposed areas into the paper or lower photopolymer layer.
Another characteristic of systems of the Chu et al. patent is that the optical density obtained upon toning depends upon temperature at the time of toning. This is because tackiness of the unexposed photopolymer systems varies significantly over the normal room temperature range of 18.degree. to 24.degree. C. This characteristic also makes it difficult to achieve proof-to-proof consistency.